This research program is designed to develop a "unified approach" for the enantioselective synthesis of the tumor promoting diterpenes, ingenol and phorbol from a common synthetic building block. The current project will also include an extension of this strategy to target the clinically important anticancer agent paclitaxel (taxol) and the unusual marine natural product, dictyolactone, a xenicane diterpene that exhibits significant antitumor activity. The focal point of the "unified" approach concept described in this proposal is the novel chromium(O)-promoted [6pi+4pi] cycloaddition process that can be employed to rapidly and efficiently construct highly functionalized bicyclo[4.4.1]undecane intermediates that can be transformed through selective bond reorganizations into key substructural features of each of the four targeted diterpene families. Methods for the direct conversion of these bicyclo[4.4.1]undecane intermediates into the BC ring substructure of the ingenane diterpenes exhibiting the crucial and highly strained inside, outside intrabridgehead stereochemical relationship will be described. Isomerization of this readily available ring system into the bicyclo[5.3.1]undecane system characteristic of the taxane diterpenes as well as rearrangement into the tigliane-like bicyclo[5.4.0]undecane system will also be discussed. In a closely related process, the nine- membered carbocycle found in the xenicane diterpenes will be prepared by rearrangement of the bicyclo[4.4.1]undecane system into an isomeric bicyclo[4.3.2]undecane followed by cleavage of the two-carbon bridge. Application of these bond reorganization protocols to the total syntheses of representative examples of each of these four families of natural products will also be presented.